Most new ASUS motherboards do not have the option to let users to turn off the UEFI secure boot.
The Secure Boot Enabled is always greyed out and unable to change that option. Yet, you do have the option to disable the secure boot by clearing secure boot keys.
This is reversible so no need to worry about breaking the warranty or damaging the BIOS.
BIOS’s Secure Boot menu should show Secure Boot state as enabled and Platform Key (PK) state as loaded
Here are the steps on how to disable the UEFI secure boot: